Direct Evidence for Shock-Powered Optical Emission in a Nova
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Direct evidence for shock-powered optical emission in a nova Elias Aydi1∗, Kirill V. Sokolovsky1;2;3∗, Laura Chomiuk1∗, Elad Steinberg4;5, Kwan Lok Li6;7, Indrek Vurm8, Brian D. Metzger4, Jay Strader1, Koji Mukai9;10, Ondrejˇ Pejcha11, Ken J. Shen12, Gregg A. Wade13, Rainer Kuschnig14, Anthony F. J. Moffat15, Herbert Pablo16, Andrzej Pigulski17, Adam Popowicz18, Werner Weiss19, Konstanze Zwintz20, Luca Izzo21, Karen R. Pollard22, Gerald Handler23, Stuart D. Ryder24, Miroslav D. Filipovic´25, Rami Z. E. Alsaberi25, Perica Manojlovic´25, Raimundo Lopes de Oliveira26;27, Frederick M. Walter28, Patrick J. Vallely29, David A. H. Buckley30, Michael J. I. Brown,31, Eamonn J. Harvey32, Adam Kawash1, Alexei Kniazev30;33;34, Christopher S. Kochanek29, Justin Linford35;36;37, Joanna Mikolajewska23, Paolo Molaro38, Marina Orio39;40, Kim L. Page41, Benjamin J. Shappee42 and Jennifer L. Sokoloski4 1Center for Data Intensive and Time Domain Astronomy, Department of Physics and As- tronomy, Michigan State University, East Lansing, MI 48824, USA 2Sternberg Astronomical Institute, Moscow State University, Universitetskii pr. 13, 119992 Moscow, Russia 3Astro Space Center of Lebedev Physical Institute, Profsoyuznaya St. 84/32, 117997 Moscow, Russia 4Columbia Astrophysics Laboratory and Department of Physics, Columbia University, New York, NY 10027, US 5Racah Institute of Physics, Hebrew University, Jerusalem 91904, Israel 6Department of Physics, UNIST, Ulsan 44919, Korea 7Institute of Astronomy, National Tsing Hua University, Hsinchu 30013, Taiwan 8Tartu Observatory, University of Tartu, Toravere˜ 61602, Tartumaa, Estonia 9CRESST and X-ray Astrophysics Laboratory, NASA/GSFC, Greenbelt, MD 20771, USA 10Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA 11Institute of Theoretical Physics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic 12Department of Astronomy and Theoretical Astrophysics Center, University of California, Berkeley, CA 94720, US 13Department of Physics and Space Science, Royal Military College of Canada, PO Box 17000, Station Forces, Kingston, ON K7K 7B4, Canada 14Institute of Communication Networks and Satellite Communications, Graz University of arXiv:2004.05562v1 [astro-ph.HE] 12 Apr 2020 Technology, Infeldgasse 12, 8010 Graz, Austria 15Dept.´ de physique, Univ. De Montreal,´ C.P. 6128, Succ. Centre-Ville, and Centre de Recherche en Astrophysique du Queebec,´ Montreeal,´ QC H3C 3J7, Canada 1 16AAVSO, 49 Bay State Rd. Cambridge, MA 02138, USA 17Instytut Astronomiczny, Uniwersytet Wrocławski, Kopernika 11, 51-622 Wrocław, Poland 18Silesian University of Technology, Institute of Electronics, Akademicka 16, 44-100 Gli- wice, Poland 19Institute for Astrophysics, University of Vienna, Tuerkenschanzstrasse 17, A-1180 Vi- enna, Austria 20Universitat¨ Innsbruck, Institut fur¨ Astro- und Teilchenphysik, Technikerstrasse 25, A- 6020 Innsbruck Austria 21DARK, Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, DK-2100 Copen- hagen Ø, Denmark 22School of Physical and Chemical Sciences, University of Canterbury, Private Bag 4800, Christchurch 8120, New Zealand 23Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, PL 00716 Warsaw, Poland 24Department of Physics and Astronomy, Macquarie University, NSW 2109, Australia 25School of Computing Engineering and Mathematics, Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia. 26Departamento de F´ısica, Universidade Federal de Sergipe, Av. Marechal Rondon, S/N, 49000-000, Sao˜ Cristov´ ao,˜ SE, Brazil 27Observatorio´ Nacional, Rua Gal. Jose´ Cristino 77, 20921-400, Rio de Janeiro, RJ, Brazil 28Dept. of Physics & Astronomy, Stony Brook University, Stony Brook, NY, USA. 29Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA 30South African Astronomical Observatory, P.O. Box 9, 7935 Observatory, South Africa 31School of Physics and Monash Centre for Astrophysic, Monash University, Clayton, Vic- toria3800, Australia 32Astrophysics Research Institute, Liverpool John Moores Univ., Liverpool, L3 5RF, UK 33Southern African Large Telescope Foundation, PO Box 9, Observatory 7935, South Africa 34Sternberg Astronomical Institute, Lomonosov Moscow State University, Universitetskii pr. 13, Moscow, 119992 Russia 35Department of Physics and Astronomy, West Virginia University, P.O. Box 6315, Mor- gantown, WV 26506, USA 36Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV 26505, USA 37National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801, USA 38INAF-Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, I-34143 Trieste, Italy 39INAF–Osservatorio di Padova, vicolo dell Osservatorio 5, I-35122 Padova, Italy 40Department of Astronomy, University of Wisconsin, 475 N. Charter Str., Madison, WI 2 53704, USA 41School of Physics & Astronomy, University of Leicester, LE17RH, UK 42Institute for Astronomy, University of Hawai’i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA 3 Classical novae are thermonuclear explosions that occur on the surfaces of white dwarf stars in interacting binary systems1. It has long been thought that the lumi- nosity of classical novae is powered by continued nuclear burning on the surface of the white dwarf after the initial runaway2. However, recent observations of GeV γ- rays from classical novae have hinted that shocks internal to the nova ejecta may dominate the nova emission. Shocks have also been suggested to power the luminos- ity of events as diverse as stellar mergers3, supernovae4, and tidal disruption events5, but observational confirmation has been lacking. Here we report simultaneous space- based optical and γ-ray observations of the 2018 nova V906 Carinae (ASASSN-18fv), revealing a remarkable series of distinct correlated flares in both bands. The optical and γ-ray flares occur simultaneously, implying a common origin in shocks. During the flares, the nova luminosity doubles, implying that the bulk of the luminosity is shock-powered. Furthermore, we detect concurrent but weak X-ray emission from deeply embedded shocks, confirming that the shock power does not appear in the X- ray band and supporting its emergence at longer wavelengths. Our data, spanning the spectrum from radio to γ-ray, provide direct evidence that shocks can power sub- stantial luminosity in classical novae and other optical transients. 1 −7 −3 In a classical nova, the accreted envelope (mass ≈ 10 − 10 M ) expands and is ejected at velocities of ∼500–5000 km s−1. The result is an optical transient where the luminosity of the system increases by a factor of ∼ 103 −106, sometimes making the source visible to the naked eye6. After the initial ejection of the envelope, residual nuclear burning continues on the surface of the hot white dwarf, leading to a phase of quasi-constant, near- Eddington luminosity powered by the hot white dwarf2, 7. This should manifest as an optical light curve smoothly declining from maximum light, as the photosphere recedes and the peak of the spectral energy distribution moves blueward from the optical into the ultraviolet and finally into soft X-ray1. However, some novae show erratic flares around maximum light with a variety of timescales and amplitudes8; these features are still poorly explored and their origin remains a matter of debate. Proposed explanations include instabilities in the envelope of the white dwarf leading to multiple ejection episodes9, 10, instabilities in an accretion disk that survived the eruption11, and variations in mass transfer from the secondary to the white dwarf12. The optical transient V906 Carinae (ASASSN-18fv) was discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN13) on 2018 March 20.3 UT, and was shortly thereafter spectroscopically confirmed as a classical nova14, 15. Serendipitously, V906 Car happened to occur in a field being monitored by the BRight Target Explorer (BRITE) nanosatellite constellation16 (Figure 1), resulting in a high cadence optical light curve track- ing the evolution of the eruption from its start (2018 March 16.13 UT; Figure 2). The 4 continuous, high cadence BRITE optical light curve (presented with 1.6 hr resolution in Figure 2, the orbital period of the satellite) revealed a series of eight post-maximum flares during the first month of the outburst, each lasting ∼ 1 – 3 days with amplitudes . 0.8 mag (Figure 2; for more details see Methods and Supplementary Information.1, hereafter SI). Typically, novae are observed using ground-based instruments at lower cadence, and light curves often contain substantial gaps, implying that such short timescale variability would be difficult to resolve. V906 Car was detected in GeV γ-rays around 23 days after eruption by the Large Area Telescope (LAT) on the Fermi Gamma-Ray Space Telescope. The γ-rays persisted at least until day 46 after eruption17 (Figure 2). The start time of the γ-ray emission is uncon- strained, as the LAT was offline during the first 23 days of the eruption. The GeV γ-ray flux reached 2:1 × 10−9 erg cm−2 s−1 on days 25 and 29, making V906 Car the bright- est γ-ray nova to date18, 19. Current theory suggests that the GeV γ-rays originate from shocks internal to the nova ejecta—specifically as a fast biconical wind slams into a slower equatorial torus20, 21. The shocks accelerate particles to relativistic speeds and γ-rays are produced when these relativistic particles interact with either